We have investigated evolutionary change in a simple repetitive region of the mitochondrial DNA genome of carnivores. Repetitive loci are abundant in nuclear genomes but less common in mitochondrial genomes. The control region is exceptional, and there are at least five loci where tandem repeats have been described in various species. We compare one locus between 21 carnivore species, representing 8 carnivore families. The sequence and organization of the repetitive motifs can differ extensively between arrays; however, all motifs appear to be derived from the core motif "ACGT." Sequence data and Southern blot analysis demonstrate extensive heteroplasmy. The general form of the array is similar between heteroplasmic variants within an individual and between individuals within a species (varying primarily in the length of the array). Within certain families, notably ursids, the array structure is also similar between species. Similarity between species was not apparent in other carnivore families, such as the mustelids, suggesting rapid changes in the organization and sequence of some arrays. The pattern of change seen within and between species suggests that a dominant mechanism involved in the evolution of these arrays is DNA slippage. A comparative analysis shows that the motifs that are being reiterated or deleted vary within and between arrays, suggesting a varying rate of DNA turnover. These data help us to understand the processes that can greatly accelerate change at a locus and create a mosaic of variation among somatic cells. The pathology of some inherited diseases, such as the fragile X syndrome, is apparently caused by a similar amplification of repetitive DNA.